AIC's 52nd Annual Meeting

In the last fifty years, awareness of preventive conservation as a holistic approach to heritage buildings has heightened. However, its actual implementation has remained slow to catch on, due in part to deferred maintenance and lack of funding facing many organizations and property owners. Additionally, thinking preventively runs counter to the reactivity of the building industry. We are usually called to respond to deterioration that has progressed to the point where it is visible. We typically perform a condition assessment, which leads to testing and treatment recommendations, and implementation of an intervention. But what do we do if there is no evidence of damage on the surface? Could we consider potential deterioration hidden within a wall assembly? Instead of reacting to what we see on the surface, could we shift our thinking to prevent deterioration before it becomes apparent?

One deterioration factor inherent in many heritage buildings is the differential durability of materials within wall assemblies. Differential durability is how the useful service life of building materials differs between components within an assembly. By considering the different durabilities and vulnerabilities of materials, we can act preemptively to anticipate potential deterioration between interior and exterior surfaces. Methods to predict the service life and durability of modern buildings are well established and have been increasingly applied to heritage buildings in the last decade. However, the results have been extremely variable and highly subjective. It is worth changing the approach to durability and service life from quantification to a comparison of relative durabilities, to understand which material is the weakest link in an assembly.

I demonstrate a methodology for qualitatively evaluating comparative durability and vulnerability in heritage buildings using two buildings as case studies. I present a decision diagram that identifies each material and its position in an assembly, assesses comparative durability of materials, identifies causal factors of deterioration and vulnerability of protective layers, and proposes interventions. While I have developed the methodology for relatively simple building envelopes, it can be applied to modern buildings or be used when designing interventions.

I focus on buildings constructed in the first half of the twentieth century when building technology and materials proliferated and designers and builders experimented with new assemblies. With the Industrial Revolution and the advent of new building materials in the late nineteenth and early twentieth centuries, exterior wall assemblies became more complicated. Architects and builders experimented with new materials, including reinforced concrete, Portland cement, and architectural terra cotta, to build multi-component walls. These walls introduced the issue of differential durability to building envelopes. Each material that comprised them had a different service life, increasing potential aging and failure points. The construction industry experimented with fabrication and installation, responding to failures by introducing new materials, changing their composition, or assembling them in different sequences. We see this same experimentation today as new materials come onto the market and buildings are designed to be increasingly weathertight. As more modern buildings become historic, the issue of differential durability will become an urgent conservation issue.